EAE552

Chapter eae552
Aircraft Maintenance Engineering
Adel A. Ghobbar
Department of Air Transport and Operations, Faculty of Aerospace Engineering, Delft University of Technology,
Kluyverweg, The Netherlands

The Aircraft Maintenance Engineering’s sections layout

1 Introduction 1 follows.
2 Summary 12
Notes 12
References 12 1.1 Airworthiness and its regulation

1.1.1 Maintenance philosophies and concepts

1 INTRODUCTION Any airline or organization that operates aircraft for trans-
porting passengers or cargo has the prime responsibility to
Everyone is looking for a way to benchmark the effective- maintain its aircraft in safe and airworthy condition. Aircraft
ness of various processes and organizations. One of the most maintenance programs vary from one operator to another. The
scrutinized areas of an airline is line maintenance effective- airlines develop their maintenance programs in accordance
ness. Someone, somewhere, decided the best measure would with their operational, commercial, and technical require-
be the “no fault found” NFF rate of components removed ments. Government regulations, route structures, availability
during the resolution of a system problem (Mobley, 2008). of manpower, and facilities are also taken into consideration
And it sounds logical, if the maintenance process were effi- (King, 1985; Friend, 1992; Kinnison, 2004). Aircraft mainte-
cient, they would remove only the components that actually nance is divided into line and base maintenance depending on
have a fault. At “Utopia Airlines,” the sky is always blue where it is performed, and into minor and major maintenance
and the flights depart on time, the mechanics work on per- depending on how long it takes to perform it.
fect problems utilizing perfect troubleshooting, and the repair Line maintenance: Line maintenance is performed at line
facility performs the perfect bench test and repair of the faulty stations or at the flight line of an airline’s base station.
components. Life is simple and straightforward, so NFF mea- Line maintenance normally consists of routine tasks with
surement is perfect for determining the effectiveness of line low intervals like servicing, cleaning, refueling, and light
maintenance. However, at “Real Life Airlines,” the sky is inspections. Nonroutine line maintenance may range from
not always blue and the flights are often late, the problems the replacement of a black box to an engine change. Line
and troubleshooting are very much imperfect, and the repair maintenance is “departure-oriented.” It will be restricted as to
facility’s bench test and repair processes are lacking. Life is the work that has to be performed and tend to defer any time-
very complicated at this and every other airline in the world. consuming items to the next base visit. The manpower and
facilities at line stations are usually limited, so maintenance
Encyclopedia of Aerospace Engineering. tasks must also be accordingly limited.
Edited by Richard Blockley and Wei Shyy Based maintenance: Based maintenance is performed at


c 2010 John Wiley & Sons, Ltd. ISBN: 978-0-470-75440-5 an airline’s maintenance base. It is “fix-oriented” because the
2 Manufacturing, Lean Technologies and Operations Integration

components during millions of flight hours and finally discov-

ered that the reliability of complex assembles (as opposed to
burn-cell items) does not decrease with increasing age. Pre-
ventive maintenance processes like HT and OC, therefore,
could not be used as a means to ensure their continued reli-
ability. The right way to maintain these complex assemblies
turned out to be a third PMP called “condition-monitoring,
CM”.
Primary maintenance process (PMP): The three PMPs
Figure 1. Airline maintenance base facility. recognized by the EASA and Federal Aviation Administra-
tion (FAA) are HT, OC, and CM. In general terms, the first
base has the manpower and facilities to perform all kinds of two both involve actions directly concerned with preventing
aircraft maintenance work, as shown in Figure 1. failure, whereas CM does not. However, the condition moni-
Minor maintenance: Minor maintenance can be performed toring process would be expected to lead to preventive action
in 24 h or less. It normally includes the routine maintenance if shown to be necessary (Civil Aviation Authority, 1990).
work up to the A-check and the resulting nonroutine tasks. These categories of component maintenance are defined as
Depending on the maintenance program in use, minor mainte- follows:
nance may also include some “C-check” tasks. It is performed Hard-time (HT): This is defined as a preventive process in
at line stations or at a maintenance base. which known deterioration of an item is limited to an accept-
Intermediate maintenance: Intermediate maintenance able level by the maintenance actions carried out periodically
comprises tasks of a “C-check” nature requiring ground times according to time in service. This time may be calendar time,
of up to seven days. Lower or higher interval tasks may be the number of cycles, or the number of landings. The pre-
included to optimize task accomplishment and the available scribed actions normally include servicing, full or partial
ground time. overhaul, and/or replacement according to the instructions in
Major or heavy maintenance: Maintenance work that relevant documentation so that the item is restored to suitable
requires an aircraft downtime of more than seven days is condition for use for a further specified period. HT requires
considered major or heavy maintenance. It includes structural that a component be overhauled after a preset usage time,
inspections and repairs, repainting, cabin refurbishment, and regardless of the component’s condition, and assumes a rela-
major modifications. Heavy maintenance is performed at a tionship between failure and age. Today, HT items rarely
maintenance base. make up more than 2% or 3% of all scheduled maintenance
tasks. Engine disks, flap links, and landing gear parts are typi-
cal HT replacement items. The individual HT are determined
1.1.2 Maintenance program development
by the manufacturer during endurance tests or by the airline
Maintenance theory: Current maintenance theory can best be as a result of its operational experience. The HT intervals
understood by taking a brief look at the historical develop- usually apply to the total life of the parts or until the overhaul
ment of aircraft maintenance theories. of the units.
During the 1930s, it was believed that mechanical parts On-condition (OC): This is also a preventive process, but
wear out, that wear-outs cause failures, and that failures one in which the item is inspected or tested at specified peri-
degrade safety. This belief led to the periodic disassembly ods to an appropriate standard to determine whether it can
of everything from engines to structures and radio sets. The continue in service. The inspection or test may reveal a need
operating time of the aircraft controlled all maintenance activ- for servicing action. The fundamental purpose of OC is to
ities. Scheduled overhaul “hard-time, HT” (as it is called remove an item before its failure in service. It is not a philos-
today) was the only recognized primary maintenance process ophy of “use until failure.” OC requires checks and tests of
(PMP). components at fixed intervals, with parts such as wires, bulbs,
Shortly after World War II, a second PMP was recog- brackets, covers, bearings, etc., being replaced during over-
nized. It was called “on-condition, OC” maintenance because haul. Typical examples of OC applications are nondestructive
it permitted the use of periodic condition checks instead of testing (NDT) tests for hydraulic and pneumatic systems,
scheduled overhauls. This process was mainly applied to borescope checks on internal engine parts, and visual inspec-
known wear items like brakes and tires. tions on structural items. It is important that these inspections
Neither of the two processes fully accommodated the new are repeated at regular intervals. The length of the intervals is
components which were introduced on the jet aircraft during fairly short at the beginning and then gradually increasing as
the 1960s. The airlines analyzed the behavior of hundreds of service experience becomes available. Some people wrongly
 Aircraft Maintenance Engineering 3

refer to units without inspection or overhaul interval as being inter-airline/manufacturer procedures for developing a main-
“on-condition” when they are not. Such units are part of the tenance program for Boeing 747 aircraft. It was subsequently
CM process. OC maintenance is like HT, a preventive main- decided that experience gained on the 747 project should be
tenance process and depends on a fixed inspection interval. applied to all newly developed aircraft. To do this, the deci-
Condition monitoring (CM): This is not a preventive pro- sion logic was updated and certain procedures specific to the
cess, having neither HT nor OC elements, but one in which 747 were deleted. That universal document resulted in MSG-
information on items is collected from operational experi- 2. In mid-1979, the Air Transport Association (ATA), with the
ence, then analyzed and interpreted on a continual basis as a intention of further updating procedures, formed a task force
means to implement corrective procedures. It is convenient to analyze MSG-2 and make recommendations for change
here to classify information, z, into two classes; namely, direct and improvements. These revisions were published by the
information and indirect information. Direct information is ATA and approved by the FAA in late 1993 as an acceptable
where z measures a variable that directly determines failure, method for developing scheduled maintenance requirements
for example the thickness of a brake pad, or the wear in a for new model transport-category aircraft (Rosenberg, 1998;
bearing. Indirect information z on the other hand provides Nakata, 1984; Brett, 1984). This is known as MSG-3.
associated information that is influenced by the component MSG-2 decision logic: Airline/Manufacturer Mainte-
condition, but is not a direct measure of the failure process, nance Program Planning Document MSG-2, dated March
for example, an oil analysis or a vibration frequency analy- 25, 1970 shows in detail how the maintenance theory is
sis. In both cases, the point of concern is to predict, given being used to determine the essential scheduled maintenance
information z, the subsequent and conditional failure time requirements for a new airplane. Systems, components, struc-
distribution as an input to modeling maintenance practice. tures, and engines are put through the MSG-2 decision logic
CM is accomplished by appropriate means available to to find:
an operator for finding and resolving problem areas. These
means range from notices of unusual problems to special 1. Which tasks can be done?
analysis of unit performance. All electronic components 2. Which tasks must be done to prevent adverse effects on
and most of the complex electrical or mechanical units are operating safety and to assure the availability of hidden
normally part of the CM process. Table 1 summarizes the functions.
overhaul control category. 3. Which tasks should be done for economic value?

The final result is a list of preventive maintenance tasks

1.1.3 The development of a maintenance steering
divided into HT and OC tasks, and another list of those items
group (MSG)
that require no scheduled maintenance and therefore included
In mid-1968, representatives of various airlines developed in the CM process.
the Handbook MSG-1, Maintenance Evaluation and Pro- MSG-3 decision logic: Airline/Manufacturer Mainte-
gram Development, which included decision logic and nance Program Planning Document MSG-3, dated October

Table 1. Overhaul control category summary.

Category Maintenance action Requirements/restrictions

Hard-time Overhaul/replace item at specified time interval Overhaul will “zero time” the item
On-condition

OC checks at specified time intervals
OC check must give reasonable assurance of
satisfactory operation until the next check

Regularly scheduled collection of OC data.
OC data must ascertain continuing airworthiness
Overhaul required when item exceeds specified and/or show reliability degradation – failure
limits for OC check or OC data imminence
Condition-monitoring (No

No scheduled overhaul or repair
Failure must have no direct adverse effect on
overhaul control) flight safety

Item is operated to failure
Hidden functions must have regularly scheduled
verification tests

Data collection/evaluation program required for
overhaul surveillance
4 Manufacturing, Lean Technologies and Operations Integration

1980, is the present-day standard to determine the essen- At this point it must be remembered that the maintenance
tial scheduled maintenance requirements for new airplanes. requirements are only one of several factors which shape
MSG-3 includes detailed decision logic for assigning tasks the form of a maintenance program. Commercial, opera-
and task intervals to all aircraft systems and components. tional, and economical needs must also be considered. The
Quite often, airlines operate a fleet of aircraft that are divided aircraft maintenance workload is generated through a contin-
between both MSG-2 and MSG-3 decision logic programs. uous airworthiness maintenance program. These programs
Generally, all new aircraft manufactured today will follow include: aircraft inspections that deal with routine inspec-
the MSG-3 concept in contrast to older aircraft continuing tion, minor services and tests performed on the aircraft at
with the older philosophy of MSG-2. Implementation of the prescribed intervals; scheduled maintenance that includes
recommendations generated through MSG-3 analysis is the replacement of life-limited items, periodic overhauls, and
major role of an airline operator in developing a maintenance special inspection; and unscheduled maintenance which is
program. The accuracy and clarity of the MSG-3 process usually generated by inspections, pilot reports, and failure
provides a smooth transition for the airline to determine analysis.
its manpower, parts, tooling, ground equipment, and other Classes of maintenance stations: From the standpoint of
related requirements. MSG-3 is based on a consistent and the maintenance function, a major carrier normally divides
rigorous application of questions for each aircraft compo- its many stations served into different classes of stations. For
nent. It is decision tree analysis at work. The first question example, in descending order of capability, they include:
MSG-3 asks is: “What’s the consequence of a specific hard-
ware/component failure for the entire aircraft?” Once this
Maintenance base
consequence is assessed, MSG-3 offers a choice of appli-
Major stations
cable tasks and evaluates each one’s effectiveness. Once a
Service stations
task is chosen, its frequency is patterned after frequencies
Other stations
adopted for similar hardware. If no comparison can be made,
a conservative frequency is initially adopted and adjusted as
experience is gained.This work resulted in the recognition of Maintenance base is generally conceded to be the largest,
a third, PMP called condition-monitoring, a process apply- most adaptable (versatile), and best-equipped facility in the
ing to components with specific design characteristics but not system. It is the overhaul and modification center for the
involving hard-time or on-condition checks. carrier’s entire fleet, and it has the capability of repairing
MSG task descriptions: The MSG-3 assigned task cate- nearly all of the components. Few components must be
gories are shown in Table 2. returned to the manufacturer or sent to outside agencies for
reconditioning.
Major stations include the carrier’s large hub cities. These
1.1.4 Aircraft maintenance scheduled programs
stations have relatively large numbers of maintenance peo-
After the MSG-2 and MSG-3 decision logic has been used ple and extensive facilities. They also maintain the largest
to determine all preventive maintenance tasks and their assignments of spare parts, mainly supplied by the mainte-
intervals, the maintenance program itself can be developed. nance base. In general, these stations are capable of rendering

Table 2. The MSG-3 assigned task categories.

Lubrication and servicing To maintain the inherent design capabilities to reduce rate of functional deterioration

Operational check Verification/certification of operation that does not require quantitative tolerances (acceptance)
Functional check Verification of an item against quantitative standard within specified limits
Restoration Replacement or restoration of an item to specified standard on or off the aircraft by cleaning or up
to overhaul. Item must show functional degradation at an identifiable age of same components
Discard Removal from service of an item at a specified life limit. The item must show functional
degradation at an identifiable age and a large proportion of the units must survive to that age
No-task Items that have no safety effect. These items follow the condition-monitoring logic per MSG-2
Combination For items that functional failure has a safety effect. When any of the previous tasks does not fulfill
requirements, a combination of tasks must be used in analyzing the proper maintenance
prerequisites
 Aircraft Maintenance Engineering 5

complete line maintenance requirements of specific types of check is a 6-12-man-hour job, depending on the type of
equipment. aircraft.
Service stations are large stations served by the carrier but 3. Service checks are performed at major, service, or desig-
not located at major hub cities with large banks of connecting nated Class 1 stations (those that are qualified to handle
flights. These stations are well equipped and well staffed them) at predetermined frequencies. The stations per-
with line maintenance personnel, but less so than the major forming this check must be approved by the EASA for
stations. the type of aircraft involved. A service check might be
Other stations throughout the system might be designated performed after an aircraft logs up to 150 h’ flight time.
Class 1, Class 2, and Class 3. A service check encompasses the items included in the
terminating preflight checks and en route service and
Class 1 stations might have only sufficient numbers of also a considerable volume of more intensive mainte-
licensed people to assure maintenance coverage for each nance. This includes inspection and servicing of cabin
flight before departure. Minimal facilities and spare parts compressors, hydraulic units, seat installations, restroom
for performing the assigned work would be provided. installation, buffet installations, cockpit equipment, inte-
Class 2 stations might have just enough mechanics and facil- rior lighting, windows, and so forth. It also includes
ities to do routine servicing, such as engine heating, examination of certain structural members, checking of
de-icing, aircraft moving, and the lightest of maintenance fuel sumps1 for water or contamination, and any special
on specific equipment. work that might be deemed necessary by the line service
Class 3 stations might include smaller cities where there engineering and line maintenance organization. A ser-
are no licensed maintenance people. They are, therefore, vice check may require as much as 35–60 man-hours, as
never scheduled to perform maintenance work and their shown in Figure 2.
aircraft servicing is limited to work that has no effect 4. A maintenance check is the most lavish attention an air-
upon airworthiness, mainly cargo and passenger han- plane gets between overhauls. Each maintenance check
dling. Ordinarily, they deal only with through trips or requires a full day to complete and it consumes from
turnaround flights. 400 man-hours on the smallest jet to 1300 man-hours on
the biggest. It covers virtually every system and com-
Types of maintenance: A carrier generally divides aircraft ponent of the aircraft. Generally only the major stations
maintenance and servicing work into different levels of inclu- are equipped to perform maintenance checks. Maximum
siveness and intensity, arranged in ascending order. Fueling times between maintenance checks might be as high as
is a separate operation, and while it can be done at the time 875 flight hours for a B-727 or 500 flight hours for a
of a check, it is not a part of the check. B-737. Maintenance checks for each type of aircraft are
planned so that the workload involved in each particular
1. En route service is performed whenever a flight operates check is about the same. The bulk of the jobs are repeated
through any of the major, service, or Class 1 stations. during every check. Some, however, are required less
It involves a visual check of the exterior (external) with frequently and thus are scheduled only for certain checks.
particular attention to indications of fuel or oil leaks and
obvious discrepancies, such as worn or flat tires, low
shock struts, or fuselage or wing damage. It may also
include interior and exterior cleaning, depending upon
need and available ground time.
2. Terminating preflight checks are performed as scheduled
when a flight terminates at a major, service, or Class
1 station, at least every 24 h. This check encompasses
the same work as en route service and also such addi-
tional items as checking of engine oil supply; checking
of engine inlet and exhaust areas for signs of deteri-
oration; checking of landing gear and tires (for wear,
inflations, fluid quantities, and so forth); checking of
exterior lighting; and checking and servicing of cabin
water system, lavatories, oxygen systems, flight recorder,
and batteries and auxiliary power units, as necessary. This Figure 2. Line maintenance service check.
6 Manufacturing, Lean Technologies and Operations Integration

Overhaul of airframes: The basic document used in for- Table 3. A, B, and C checks intervals.
mulating the overhaul plan of a major carrier’s jet fleet is the
work report prepared by the engineering department for main- Interval Flight hours Ground time
taining the structural integrity of the particular aircraft. When
“A” check 200 to 500 8–16 h
this document is approved by the EASA, it becomes a part of
“B” check 600 to 1500 16–36 h
the operations specifications, which detail the requirements “C” check 2000 to 5000 48 h and more
for continuous airworthiness. Compliance is not only desir-
able but also mandatory. A separate work report covers the
entire structure, the landing gear, and all control surfaces of inspections are performed. The total package is called a “D
each jet aircraft type by zones. It spells out the kind of inspec- check” or if the package content is varying, a “heavy main-
tion each item is to receive and designates the frequency or tenance visit.”
interval of inspections. Nonroutine maintenance: There is rule of thumb that
Overhaul of engines and other components: In general, claims that each hour of routine maintenance generates one
overhauls of engines, their accessories, and other compo- hour of nonroutine maintenance. While this is not always
nents are handled in much the same manner. Components are true, it is true enough to serve as an indication of how many
brought in when either operating time or condition requires nonroutine maintenance hours can normally be expected.
it, and the overhaul returns them to specifications laid down Nonroutine maintenance results from malfunctions, pilot
by engineering and the manufacturer. A large part of engine complaints, and maintenance squawks/complain. Aircraft
overhaul is made up of repair and reconditioning operations, modifications are another source of nonroutine maintenance
as it is usually beneficial, both economically and from the work. Depending on the size and the urgency of the modifi-
standpoint of reliability, to reuse seasoned/tested components cations, they will be performed together with any scheduled
when they can be reworked to approved specifications. Sched- check or in some case during an extended special aircraft
uled engine changes are planned so as to minimize shipping downtime.
costs and transit times and avoid special routing of aircraft. Modification to aircraft: “Modification” means the alter-
All scheduled big-jet engine changes are handled at the car- ation of an aircraft/aircraft component in conformity with
rier’s major base, and all others at compromise locations an approved standard. Modifications are changes to aircraft
where routing is convenient and labor is available. When and their equipment or substitution of parts. All modifica-
practical, engine changes are made during maintenance check tions affecting airworthiness must be approved by the EASA.
or airframe overhaul. Modifications often involve design changes; aircraft opera-
Type of aircraft overhaul checks: To conform with EASA tors possess limited design authority, so the modification is
and FAA guidelines, some companies have adopted mainte- usually devised by the manufacturer. The European EASA
nance policies that call for routine inspections at least every classes modifications as either minor or major:
four days. The first major check (denoted as “A” check), man-
dated by the EASA, occurs every 200 flight hours, or about Minor modifications regarded by the EASA as relatively
once a week. “A” checks involve a visual inspection of all unimportant may be designed, embodied, and certified
major systems such as landing gear, engines, and control by an approved organization or licensed aircraft main-
surfaces. “B” checks are performed every 600–1500 flight tenance engineer. Minor modifications, which are more
hours, and entail a thorough visual inspection and also lubri- important, require approval by the EASA directly or by
cation of all moving parts such as horizontal stabilizers and an approved design organization. Modification approval
ailerons. “C” and “D” checks are done about once every one requires a form detailing the modification, its applica-
to four years, respectively, and require taking the aircraft out bility, and relevant drawings or specifications. A civil
of service for up to a month at a time. Starting with an “A” modification record or work done must be kept; the
check, each higher check type demands more ground time EASA may inspect this as required.
and is performed less frequently, as shown in Table 3. Inter- Major modifications require a more extensive approval
vals and ground time vary considerably depending on the process by the EASA including a certificate of design
type of aircraft and the type of operations involved. (modification) and the issue of an airworthiness approval
“D” check or heavy maintenance visit: After a number of note (AAN) by the EASA. A civil modifications record
years of operation, some major work needs to be done on of these must also be kept.
transport aircraft. During a downtime of anywhere from four
to eight weeks such major tasks as paint renewal, cabin refur- Contract maintenance: Many of the newer carriers do not
bishment, control surface removals, and internal structural have a maintenance base capable of performing major aircraft
 Aircraft Maintenance Engineering 7

overhauls. Maintenance by their own personnel is often lim- Certificate of maintenance review (CMR): A valid cer-
ited to en route service and terminating preflight checks. All tificate of maintenance review states the date on which the
major maintenance is contracted with a major or national review was carried out and the date on which the next review
carrier and performed at the latter’s maintenance base or at a is due as specified in the approved maintenance schedule or
major station where equipment and personnel are available. associated approved document. The certificate records the
Also, it is not uncommon for carriers of all sizes to contract current maintenance status of the aircraft against the mainte-
maintenance service work either with another carrier or with nance schedule and any modifications or inspections required
a private maintenance firm at those stations that receive a by EASA/FAA. The issue of certificates of release to service
limited number of flights. and all technical log defects, including the rectification or
Subcontracting maintenance: Many airlines contract out deferment of these defects, are also noted.
work, which may be more economically performed either by Certificate of release to service (CRS): An aircraft regis-
manufacturers, other airlines, or overhaul agencies – because tered in the local authority and issued with a C of A shall not
the capital investment in test facilities, manpower, and accom- fly without a certificate of release to service being issued the
modation may not be justifiable within the airline for the following:
volume of components in the fleet and those held as spares.
Equally, when justifiable, airlines may set up such facilities

Overhauls, repairs, replacements, modifications, and
as required, and perform the work for other airlines. mandatory inspections.
Contract and pool maintenance: Maintenance and pool

Work on radio equipment or survival craft done on the
contracts refer to agreements between airlines to do mainte- aircraft.
nance for each other. Airlines with low frequency of flights
into some of their stations should consider contract or pool The CRS as issued must be signed in all the relevant
maintenance for these stations. Although the cost per main- work categories for the work done. Those signing must be
tenance man-hour and material will be higher for contracted satisfied that all work has been properly carried out using
maintenance, the total direct maintenance cost per airplane the correct and up-to-date manuals, drawings, specifications,
flight hour may be lower. This is especially true for smaller recommended tools and test gear, and in a suitable working
airlines and airlines with a lengthy/infrequent route structure. environment.
Currently some airlines have agreements whereby one air- Authorized release certificate (form EASA 1): This is an
line may do the entire engine overhaul, another airline will internationally recognized import/export parts release docu-
overhaul most of the components, and a third airline will ment issued by organizations specifically approved for the
accomplish all the structural inspection. purpose. The certificate signifies that the part has been man-
ufactured or overhauled in compliance with the requirements
and specifications called for by the aircraft constructor. It is
1.1.5 Aircraft maintenance certificates
the duty of an operator to verify that all parts received have
Certificate of airworthiness (C of A): An aircraft must have a the appropriate release documents.
current C of A to fly; this is issued under the law of the country Certification of modifications: All modification incor-
in which the aircraft is registered. The C of A becomes invalid poration work must be supervised either by an approved
if the aircraft is “repaired, replaced, removed, overhauled, or organization or by a licensed aircraft maintenance engineer
modified in manner not approved by the regulator, or any in the appropriate category. Before a certificate of release to
inspection, or modification classified as mandatory by the service is issued, it should be confirmed that the correct draw-
regulator that ensures the aircraft remains airworthy has not ings and literature for the modification have been used and
been carried out. When the C of A is issued, the category of that the modification has been correctly carried out, tested,
use is also specified. For large civil aircraft this will normally and inspected.
be transport category (passenger/cargo); thus a cargo aircraft
is not allowed to carry passengers.
1.1.6 Applications of European aviation safety
The air operator’s certificate (AOC): Air operator’s cer-
agency (EASA)
tificate is granted by the EASA/FAA. This signifies that the
holder is considered to be capable to secure the safe operation EASA Part-OPS: Operator’s responsibility is to ensure the air-
of the specific aircraft detailed on the AOC. For this purpose, worthiness (Joint Aviation Authorities, 1991) of the airplane
the local regulator evaluates the organization, staff, training, by:
equipment, and maintenance practices of the operator (Civil
Aviation Authority, 1992).

Accomplishment of preflight inspections
8 Manufacturing, Lean Technologies and Operations Integration

Rectification to an approved standard of any defect and

damage affecting safe operation

All maintenance in accordance with approved program

Analysis of the effectiveness of the maintenance

Accomplishment of operational or airworthiness direc-
tives and mandatory requirements

Embodiment of modifications in accordance with an
approved standard and for nonmandatory modifications,
the establishment of an embodiment policy

Ensure that the C of A for each airplane operated remains
valid

Approved maintenance organizations, EASA Part-145 is

to specify the requirements for organizations that maintain Figure 3. Airline spare parts warehouse.
commercially used aircraft and components of such air-
craft. Many maintenance organizations have already obtained
When focusing research efforts on aircraft component
EASA Part-145 approval from their local EASA authority.
maintenance, airlines must determine the method of repair
Quite often this was not a simple process. The requirements
required to lower cost, maintain quality, and keep aircraft
are not always clear and they need to be translated to the prac-
utilization high. Quite simply, when an aircraft component
tical situation within the company. Also, the requirements
is removed for maintenance, it will usually be replaced on
change regularly.
the aircraft by a like component taken from stock. Hav-
Approved of maintenance training organizations, EASA
ing the component in stock will allow the aircraft to be
Part-147: It specifies or prescribes the requirements to be
returned to service quickly but will cause measurable cost
met by organizations seeking approval to conduct approved
to be incurred. Whether from an in-house shop or a mainte-
training/examination of certifying staff, as specified in EASA
nance vendor, the quicker and more predictably the spares are
Part-66.
repaired and returned, the fewer the component spares that
Approved of certifying staff EASA Part-66 (engineer
need to be carried in stock while at the same time not incur-
licensing): EASA rule introducing qualification requirements
ring the probability of an out-of-stock situation as shown in
for certifying staff. Certifying staff are those personnel autho-
Figure 3.
rized to release an aircraft to service after maintenance work
The aircraft spare-parts industry is unsure of itself. While
in EASA Part-145 approved maintenance organization.
some companies enjoy growth, others face uncertain futures
and many no longer see the sense in sitting on huge invento-
ries of spare parts when they can turn those assets into cash
1.1.7 Aircraft parts management
or task out storage and upkeep to others. The airlines indus-
Maintenance inventory systems: Service parts are a major try, having risen rapidly in the last few years from the floor
expense for companies in all sectors, one that often of recession, has been seeking alternative ways to manage
exceeds annual profits. The commercial aviation industry, its spare parts and overhaul shops, whereas start-up airlines
for instance, holds an estimated $45 billion in spare parts have been seeking a complete service in this area. There is a
worldwide. A conservative estimate for the cost of holding huge, pent-up demand by airlines who want their spare-parts
this inventory is $6.1 billion per year, more than four times inventories managed in a more efficient way. Most airlines
the combined profits of the world’s airlines between 1995 and have far more inventory than they need. The past view of “if
1997, and even this is probably understated. A reduction in we need it, get three,” became extremely expensive and in
inventory by operators could free up huge amounts of capital their effort to be masters of their own destinies, and in their
and reduce operating expenses. over-riding fear of an aircraft being late, the airlines built up a
Maintenance and inventory control problems have been a 10-year supply of spare parts. However, overheads like pay-
focus area for many who work in the area of airline operations ing for property taxes, storage costs, heat, lighting, together
research; of particular interest is the application of quantita- with their management, just to keep spares on the shelf prob-
tive techniques to control the cost of component maintenance, ably add 30% a year to the price of the part; so all airlines are
or repairable parts, and to improve overall equipment utiliza- now focused on reducing their inventory stock. For this rea-
tion. son there are several challenges facing the airline industry that
 Aircraft Maintenance Engineering 9

70

60 59

ROP
50
MRP
Number of companies

40
35

30 29

20
20
13
10 9 8

1 1 0
0
Europe Middle East, Africa Latin America, Caribbean North America Asia, Pacific
World continents

Figure 4. The implementation of MRP and ROP systems by continent.

make the use of more sophisticated inventory control systems priate for such items. The issue of intermittent (Williams,
essential. The chronic unreliability of historical data, rapidly 1984; Campbell, 1963) demand in inventory management
changing products, equipment, fleets of aircraft that are aging is too often neglected. Handbooks describe ROP based on
and the introduction of regulatory changes are all impacting normal demand distribution. Many other standard software
on the reliability of traditional data. packages for inventory management and control also take
For various reasons however, most manufacturers, espe- normality of demand for granted. We take issue with this.
cially smaller ones, have clung to older methods of reordering With inventory problems arising when periodic overhauls
component-parts. The method favored is usually some form are scheduled, how many spare parts should be stocked at
of the traditional reorder point system (ROP) (Ghobbar and the aircraft maintenance center to meet demands, and as
Friend, 1996), in combination with a bill of materials explo- the demand requirement becomes intermittent, how effec-
sion (BOM. ROP plans for routine replenishment of parts, tive is it to use such classical inventory methods as the ROP
whereas the BOM explosion is performed for the purpose method? Of late, we have seen airlines start moving toward
of generating parts-shortage lists for components needed in a system preferred by many aerospace manufacturers, the
support of the current master schedule (MS). Since ROP con- materials requirements planning (MRP) system (Ghobbar
tinues to be in wide use in the aviation industry, it warrants and Friend, 2004; Friend and Ghobbar, 1999), as shown in
the attention we give it, yet the deficiency of ROP for plan- Figures 4 and 5.
ning dependent-demand inventories has been demonstrated Parts classification: Before computers, paying equal atten-
by many operators. The ROP is deficient in that it results in tion to all inventory parts was not feasible. The focus would
an unnecessary excess of parts or stockouts. be on a few expensive and fast-moving parts. Parts were cat-
Few companies can boast that they achieve the maximum egorized into homogeneous groups based on their particular
benefits from their parts investments that are notoriously dif- characteristics. This is the principle of selective inventory
ficult to manage. Demand is variable and hard to predict, lead control. Several procedures for classifying parts into homo-
times for replenishment parts are often erratic, stockout costs geneous groups are available, a few of which have been listed
are difficult to measure, and the lot size of part inventories in Table 4.
usually requires large order quantities. Several noncost criteria have been identified as important
Unpredictable parts (Ghobbar and Friend, 2002), which in the management of maintenance inventories. Among them
form sporadic or intermittent2 demand patterns with highly are lead-time, obsolescence, availability, substitutability, and
skewed distributions, are common in parts inventory, and criticality. In discussions with managers, this final concept of
much available inventory control methodology is not appro- criticality seemed to sum up their feelings about most aspects
10 Manufacturing, Lean Technologies and Operations Integration

120

100 96

ROP
Number of companies

80 MRP

60 56

40

20 16
7
0
Airline companies Maintenance organizations
Type of operation

Figure 5. The implementation of MRP and ROP systems by type of operation.

Table 4. List of selective inventory control procedures. Table 5. Inventory types and classifications.

Technique Description Basis for formulation Class Type Description

ABC Pareto rule Annual usage value of A Spares Aircraft rotables (serialized and
the parts life-controlled)
VED Vital, essential and Criticality of the parts B Spares Aircraft repairables, ground
desirable equipment, components
FSN Fast, slow and Usage rate of the parts C Spares Aircraft expendables, ground
nonmoving equipment, consumables
HML High, medium, low Unit price of the parts C Maintenance Standard items (bolts, nuts, washers,
SDE Scarce, difficult, and Procurement stocks etc.)
easy to procure lead-times C Maintenance Sealants, paints, oils, and greases
stocks
C Raw materials Bulk items (carpets, sheet metals,
vinyls, etc.)
of maintenance items. It takes into account such factors as C General supplies Commercial items (rags, detergents,
the severity of the impact of running out, how quickly the inspection fluids)
item could be purchased and whether there was an available C Tools Aircraft tools, commercial tools
substitute. It remained to be seen whether it would be feasible
to distinguish degrees of criticality in practice.
The purpose of spares classification is to provide quick aircraft while still maintaining the desired level of safety at
identification of aircraft spares either within a single airline or the time of despatch on revenue operations while operating
in conjunction with other airlines (Smith and Golden, 1991). within a controlled and sound program of repairs, replace-
To provide material support for the maintenance and over- ment, and servicing.
haul of such a variety of equipment, over a million items The MMEL is a list that may be produced by the aircraft
are stocked (Pfunder, 1986). These items are classified under manufacturer or by the airworthiness authority (CAA/JAA)
rotables, (Class A), Repairables, (Class B), and expendables, and covers all aircraft of a specified type. This is described
(Class C). The inventory parts classifications are shown in more fully in CAP-549 (Civil Aviation Authority, 1990).
Table 5. For an operator to develop a MEL applicable solely to his
Master minimum equipment list (MMEL): This is a list of or her own operation, he or she must use MMEL. The MEL
items that are permitted to be temporarily inoperative on an must be no less restrictive than the applicable MMEL but may
 Aircraft Maintenance Engineering 11

include additional advisory material and define any additional cited problems in the development of reliable forecasts owing
or modified operational procedures. MEL determination is to the relatively high percentage of items that experienced
regarded as a matter principally for flight operation depart- erratic or lumpy demand.
ments with engineering input. Engineering has responsibility
for ensuring that aircraft are kept to MEL standards. This may
1.1.8 Aging aircraft maintenance
be known to some operators as criticality and/or essentiality
that can be categorized into three codes as follows: Structural integrity evaluation methods for aging aircraft are
surveyed. Design approaches and programs adopted by the
1. A flight cannot be dispatched for commercial service aircraft industry to ensure structural safety are reviewed.
with the part inoperative. Corrosion fatigue is the major failure mechanism affect-
2. A flight can sometimes be dispatched for commercial ing airframe integrity. Therefore, corrosion fatigue reliability
service with the part inoperative. analysis and life prediction methods are emphasized in detail.
3. A flight can always be dispatched for commercial service Probabilistic methods to simulate the total corrosion fatigue
with the part inoperative. damage process and to estimate the reliability, considering
both single-site and multiple-site damage, are implemented.
There are several ways by which the criticality of a part In recent years, a number of research studies have been con-
can be defined. A part may be called critical if the loss of ducted worldwide in the field of aging aircraft and their
operation caused by nonavailability of the part is very high. attendant problems. Statistics analysis showed that 31% of
If a substitute part is readily available, then the part may be the US aircraft fleet exceeded the design goals, and that by
less critical. the year 2000 about 60% of the worldwide fleet of US manu-
Components maintenance forecasting: Demand forecast- factured aircraft would be 20 or more years old (Mahadevan
ing is one of the most crucial issues of inventory management. and Shi, 2001). It, therefore, became necessary to reevalu-
Forecasts, which form the basis for the planning of inventory ate the service life of the large fleet of aging aircraft, based
levels, are probably the biggest challenge in the repair and on their condition and service environment. To ensure air-
overhaul industry, as the one common problem facing air- craft safety, the aircraft industry has developed several design
lines throughout the world is the need to know the short-term philosophies, such as the safe-life design approach, fail-safe
part demand forecast with the highest possible degree of accu- approach, and damage tolerance approach. Some programs
racy. The high cost of modern aircraft and the expense of such have also been developed to ensure the structural integrity
repairable spares as aircraft engines and avionics constitute from the maintenance and management perspective. From the
a large part of the total investment of many airline operators. perspective of the failure mechanism of the aircraft structures,
These parts, though low in demand, are critical to opera- fatigue, and corrosion are of serious concern (Goranson,
tions and their unavailability can lead to excessive downtime 1997). Corrosion leading to fatigue crack nucleation and
costs. Most airline materials managers deal with intermittent crack growth is considered to be among the most signifi-
demand, which tends to be random and has a large propor- cant degradation mechanisms in aging aircraft. Especially,
tion of zero values. This topic has received extremely limited widespread corrosion pits on the surface and hidden within
study within the aviation industry (Ghobbar and Friend, 2003; the fuselage joints are an important cause for multiple-site
Salamanca and Quiroz, 2005; Adams, Abell and Isaacson, damage, because fatigue cracks are observed to nucleate and
1993). propagate from these corrosion pits. Some mechanics-based
Forecasting the demand for parts with highly variable models for the crack nucleation and propagation initiated
demand patterns is one of our main objectives because some from corrosion have been developed for corrosion fatigue
of the traditional forecasting methods generate results with life prediction.
such large error margins that, in many cases, as they cre- Many design considerations are involved in ensuring
ate too many stockouts managers find them useless. Airline structural integrity of Boeing and Airbus jet transports,
operators and maintenance service organizations have sev- which have common design features validated by extensive
eral common problems that were mentioned in relation to analyses, tests, and service performance. Designing for con-
developing service part forecasts. Firstly, it showed that most tinued structural integrity in the presence of damage such
companies felt the service part forecasts they received were as fatigue or corrosion is an evolutionary process. Perfor-
never realistic and as such they tried to outguess the forecast. mance demands, increasing structural complexity, and aging
Secondly, for those companies that did implement the MRP fleet reassessments have required development of standards
system, the service part forecast was loaded directly into the suitable for application by large teams of engineers. New
system without any review; and finally, firms most commonly developed methods and with special emphasis on practical
12 Manufacturing, Lean Technologies and Operations Integration

fatigue reliability considerations shown to be an effective tool The fact that nearly every influential textbook on air-
to monitor fatigue and corrosion at earlier stage (Yanjie and craft’s maintenance management embodies the Bath-Tub
Zhigang, 2003). Durability evaluations are based on quan- Curve indicates that there is an enigmatical trust in the rela-
titative structural fatigue ratings that incorporate reliability tion between failure and life. However, seen from the angle
considerations for test data reduction and fleet performance of maintenance, these models are full of practical difficul-
predictions. Fatigue damage detection assessments are based ties and contradictions. What exactly is the relation between
on detection reliability estimates coupled to damage growth maintenance and product life? This makes professionals
and residual strength evaluations. Data are presented to air- obscure and puzzled, because there lacks a typical main-
line operators on detection check forms that permit efficient tenance model, which reveals product’s life features from
maintenance planning to achieve required fatigue damage the point of view of maintainability. Modern aircraft and
detection reliability levels. most industrial undertakings consist of thousands of differ-
Criteria and procedures used in commercial jet trans- ent components, accessories, and equipment, whereas each
port design and manufacture over the last four decades have component, accessory, and equipment is assembled with dif-
resulted in fail-safe/damage-tolerant structures with a credi- ferent basic elements. Different components, accessories, and
ble safety record. Advancements in the capability to charac- equipments of an aircraft have different reliability perfor-
terize structural performance by analysis have spurred adap- mances. The combination of such complicity and variety
tation of traditional fatigue and fracture mechanics technolo- means that it is not possible at all to make a whole analyt-
gies with large test and service databases to achieve develop- ical account of the whole aircraft, or even of the reliability
ment of technology standards over the last two decades. performances of its main equipment. The optimum inspec-
tion frequency is determined by minimizing the expected
cost while the constraint on the specified fleet reliability is
2 SUMMARY satisfied. It has been shown that the optimum inspection fre-
quency increases as the relative importance of the cost of
Maintenance of aircraft is a complex process that has consol- inspection compared with the cost of failure becomes smaller,
idated over the past decades significantly. This complexity thus increasing the fleet reliability more significantly. Though
combined with all the safety and reliability issues related great progress has been made in the area of aging aircraft,
to it have made this process difficult to modify. However, the continued desire to maintain aircraft in revenue service
this complexity should not prevent one from continuously beyond their design service objectives will almost certainly
questioning the different steps performed with respect to result in new structural integrity and systems reliability prob-
advanced technology being provided. The variety of activities lems. It is the mission of the FAA and EASA’s Aging Aircraft
mentioned in this chapter with regard to designing, moni- Program (or perhaps more appropriately the Continued Air-
toring, and managing fatigue or deterioration in general of worthiness Program) to ensure that age-related problems are
aircraft and specifically their structures show that structural predicted and eliminated or mitigated before their having a
integrity combined with aging issues is a major concern. A major impact on safety.
broader thinking in terms of life-cycle cost has become highly
important. Advanced sensing, materials, and data processing
technology are possibly the biggest challenges to be seen NOTES
in revolutionizing the inspection process and in automating
the procedure leading to the most common information of 1. A hole or container, especially in the lower part of an
“no failure found.” The life profile of an aircraft is a unity engine, into which a liquid that is not needed can flow.
of opposites. It can be thought of as a conflicting process 2. Intermittent demand is synonymous with terms such as
between failure process and maintenance process, and the lumpy, sporadic and erratic demand. Throughout the the-
end of this contradiction always means the end of life for sis, we come across these words.
the aircraft. Statistical analysis to find out maintenance ratio
curve, expecting to recognize macroscopically the basic law
of maintenance process, and then to proceed to the next step
with the aim of controlling the consumption of maintenance REFERENCES
man-hours resources so that it will play a positive role in
raising the level of maintenance and management of modern Adams, J.L., Abell, J.B. and Isaacson, K.E. (1993) Modelling and
aircraft. Thus it makes our new recognition of the aircraft Forecasting the Demand for Aircraft Recoverable Spare Parts,
maintenance and management. RAND, Santa Monica, CA, USA.
 Aircraft Maintenance Engineering 13

Brett, L.F. (1984) Integration of MSG-3 into airline operation, in Goranson, U.G. (1997) Fatigue issues in aircraft maintenance and
Aerospace Congress and Exposition, SAE, Long Beach, CA, repairs. Int. J. Fatigue, 20(6), 413–431.
USA, paper no. 841483. Joint Aviation Authorities (1991) Joint Aviation Requirements:
Campbell, H.S. (1963) The Relationship of Resource Demands to Approved Maintenance Organisations, CAA, JAR-145, London,
Air Base Operations, RAND, Santa Monica, CA. UK.
Civil Aviation Authority. (1990) Condition Monitored Mainte- King, F.H. (1985) Aviation Maintenance Management, Southern
nance: An Explanatory Handbook, CAA, CAP 418, London, Illinois University Press, Carbondale, USA.
UK. Kinnison, H.A. (2004) Aviation Maintenance Management,
Civil Aviation Authority. (1990) Master Minimum Equipment Lists, McGraw-Hill, New York, USA.
MMEL, and Minimum Equipment Lists, MEL, CAA, CAP 549, Mahadevan, S. and Shi, P. (2001) Corrosion fatigue reliabilityof
London, UK. aging aircraft structures. Prog. Struct. Eng. Mater., 3(2), 188–197.
Civil Aviation Authority. (1992) Air Operators’ Certificates: Part Mobley, R.K. (2008) Maintenance Engineering Handbook,
Two Arrangements for Maintenance Support, CAA, CAP 360, McGraw-Hill, New York, USA.
London, UK.
Nakata, D. (1984) An introduction to MSG-3, in Aerospace
Friend, C.H. (1992) Aircraft Maintenance Management, Longman Congress and Exposition, SAE, Long Beach, CA, USA, paper
Scientific & Technical, Harlow, UK. no. 841481.
Friend, C.H. and Ghobbar, A.A. (1999) Extending visual basic for Pfunder, K.R. (1986) Selecting the right approach to establish MRO
applications to MRP: Low budget spreadsheet alternatives in stocking levels. APICS – Fall Seminar Proceedings, pp. 103–111.
aircraft maintenance. Prod. Inventory Manage. J., 40(4), 9–20.
Rosenberg, B. (1998) Business jet operators adopt MSG-3 philos-
Ghobbar, A.A. and Friend, C.H. (1996) Aircraft maintenance and ophy. Overhaul and Maintenance, 4(3), 39–42.
inventory control using the reorder point system. Int. J. Prod.
Res., 34(10), 2863–2878. Salamanca, H.E. and Quiroz, L.L. (2005) A simple method of esti-
mating the maintenance cost of airframes. Int. J. Aircraft Eng.
Ghobbar, A.A. and Friend, C.H. (2002) Sources of intermittent Aerosp. Technol., 77(2), 148–151.
demand for aircraft spare parts within airline operations. J. Air
Transp. Manage., 8(4), 23–33. Smith, J.R. and Golden, P.A. (1991) Airline: A Strategic Manage-
ment Simulation, Prentice-Hall, Englewood Cliffs, NJ, USA.
Ghobbar, A.A. and Friend, C.H. (2003) An evaluation of forecasting
methods for intermittent parts demand in the field of aviation: A Williams, T.M. (1984) Stock control with sporadic and slow-moving
predictive model. Comput. Oper. Res., 30(14), 2097–2114. demand. J. Oper. Res. Soc., 35(10), 939–948.
Ghobbar, A.A. and Friend, C.H. (2004) The material requirements Yanjie, Q. and Zhigang, L. (2003) New concept for aircraft
planning system for aircraft maintenance and inventory control: maintenance management. Proceedings Annual Reliability and
a note. J. Air Transp. Manage., 10(4), 217–221. Maintainability Symposium.
Abstract:
The airframe maintenance cost, taking into account nondestructive inspections, safety, and the repair and replacement of
crack components, has a high priority in the practical fleet management of airplanes. In technical terms, the service lifetime
of an aircraft can be extended for a long period by performing inspections, repairs, and making feasible replacements with
appropriate frequency. However, maintenance costs tend to increase and prolonging aircraft life can become uneconomic, as
can the management of older fleets. A repairable or maintained system is a system that, after failing to perform one or more
of its functions satisfactorily, can be restored to fully satisfactory performance by any method, other than replacement of the
entire system. Effective and efficient maintenance management is essential not only for production systems but also for large-
scale service systems, such as air and surface transport systems. These repairable systems are subject to aging mechanisms
such as wear, fatigue, creep, and stress corrosion. Inspection and diagnostic activities are integral components of an effective
maintenance strategy in an attempt to ensure system safety, reliability, and availability.
Keywords: aircraft maintenance, reliability engineering, maintenance cost, aging aircraft, airline maintenance planning,
maintenance steering group (MSG), maintenance inventory control